Trucks, particularly heavy duty trucks, commonly employ multiple speed counter-shaft type mechanical transmissions having up to at least 18 different torque multiplication ratios. The large number of ratios is needed to enable a fully loaded truck to perform a variety of necessary tasks, including low speed maneuvering in forward and reverse gears as required for moving about freight yards and for loading and unloading tasks, accelerating from a dead stop, accelerating while rolling, maintaining speed while on a grade, and so on. The large number of gears means that there is a frequent need for shifting. Manual gear shifting, as well as the selection of the correct gear, are tasks which require considerable operator experience to consistently execute optimally. Increasingly, transmissions for heavy-duty trucks are being automated. However, even with a large number of ratios, and automated shifting, the engine speed varies with the speed of the vehicle when the vehicle is in a given gear ratio. This means that the engine's operating parameters must be compromised to accommodate the anticipated range of engine operating speeds. An engine that could be tuned to operate at a single engine speed could be tuned to operate much more efficiently. Shifting decreases the operating efficiency of the vehicle, as there can is typically a dip in vehicle speed during the shift when the engine is momentarily disconnected from the drive wheels, and a subsequent need to bring the vehicle back to its target speed.
It is desired to provide a transmission which provides a full ratio coverage, yet minimizes the need for shifting. It is also desirable to provide the engine with a narrower anticipated speed operating range so as to permit the optimization of engine parameters. One of the limitations to the use of CVTs has been their limited torque capacity. One approach to dealing with the relatively low torque capacity of CVTs is to reduce the torque sustained by the CVTs. This has been achieved by splitting the drive torque from the engine into two components, with only part of the torque going through the CVT. A planetary system is commonly used to recombine the CVT and direct torques. The result of such a power splitting arrangement is that, while it provides a relatively high torque capacity CVT system, that system has a relatively narrow torque multiplication ratio band. The narrow ratio concern has been overcome by combining the CVT power splitting system with a multi-step ratio transmission. One such system is illustrated in U.S. Pat. No. 5,167,591, the teachings of which are hereby included by reference. It illustrates the use of a torque splitting planetary arrangement in combination with a stepped transmission. However, given the high torque outputs of engines used for heavy duty trucks, and the limited torque capacities of know CVT systems, particularly CVT belt-type systems, further torque reduction was necessary. It is also desired to provide a compact packaging arrangement for such a system.